Microporous polyolefin membranes are widely used for various applications such as battery separators used for lithium batteries, etc., various capacitor separators, various filters, moisture-permeable, waterproof clothes, reverse osmosis filtration membranes, ultrafiltration membranes, microfiltration membranes, etc.
Separators for lithium secondary batteries and lithium ion battery batteries are required not only to close fine pores to stop the battery reaction when abnormal heat generation occurs by the short-circuiting of external circuits, overcharge, etc., thereby preventing the heat generation, ignition, explosion, etc. of the batteries, but also to keep their shapes even at high temperatures to prevent the dangerous direct reaction of positive electrode materials with negative electrode materials. However, microporous polyolefin membranes widely used as separators at present, which are stretched in the production process, do not have high shape retention properties at high temperatures.
As portable electronic equipments and laptop personal computers have got smaller and lighter in weight, lithium ion secondary batteries have been made thinner with higher capacity, and to prevent the reduction of battery capacity, short-circuiting between electrodes, decrease in cycle performance, etc., which would occur as a result of the size reduction of the batteries, separators are required to have improved adhesion to electrodes. With respect to this point, too, however, conventional microporous polyolefin membranes are not satisfactory.
As a separator excellent in both ion conductivity and adhesion to electrodes, JP2001-118558A proposed a separator for lithium ion secondary batteries, which comprises ion-conductive polymer layers having a thickness of 5 μm or less dispersed on at least one surface of a microporous polyolefin membrane at a surface-covering ratio of 50% or less. However, it may be difficult to control pore diameters of the ion-conductive polymer layers, because this separator is produced by simply coating at least one surface of the microporous polyolefin membrane with an ion-conductive polymer solution and drying it. As a result, the separator is likely to have insufficient permeability.
As a separator having shutdown properties and an electrolytic solution retention, JP2002-216734A proposed a lithium battery separator constituted by a three-layer microporous membrane comprising microporous surface layers made of a vinylidene fluoride-containing copolymer having a melting point of 145° C. or lower, and a microporous intermediate layer made of a polyolefin having a melting point of 140° C. or lower. This separator is produced by (1) a method of forming a microporous membrane made of a vinylidene fluoride-containing copolymer and a microporous polyolefin membrane in advance, overlapping them, and stretching and press-bonding them, or (2) a method of simultaneously extruding solutions of the above polymers, cooling them to cause phase separation to form a three-layer sheet, and stretching it after removing a membrane-forming solvent, or removing a membrane-forming solvent after stretching it. However, fine pores are likely to be closed by the press-bonding in the above method (1), and stretching should be conducted at a high magnification to obtain a high-permeability membrane in the above method (2), thereby failing to achieve good heat shrinkage resistance. Further, any of the above methods (1) and (2) suffers the problem that peeling is likely to occur unless the microporous membrane layers have melting points close to each other.
As a microporous membrane with improved absorption of a electrolytic solution in the production process of batteries, improved cycle properties, etc., the applicant previously proposed a composite membrane comprising a porous coating layer of a gelable functional polymer formed on at least one surface of a microporous polyolefin membrane, the porous coating layer having an average diameter larger than the maximum pore diameter of the microporous polyolefin membrane (JP2002-240215A). This composite membrane is produced by (1) a method of coating at least one surface of the microporous polyolefin membrane with a polymer dissolved in a good solvent, immersing the coated microporous membrane in a poor solvent to cause phase separation, and drying it, (2) a method of coating at least one surface of the microporous polyolefin membrane with a polymer dissolved in a mixed solvent of a good solvent and a poor solvent, evaporating the good solvent selectively to cause phase separation, and removing the remaining solvent, or (3) a method of coating at least one surface of the microporous polyolefin membrane with a polymer dissolved in a good solvent, cooling it to cause phase separation, and drying it.
It has been found, however, that because immersion in a poor solvent is conducted in the above method (1), the resultant composite membrane is likely to suffer the peeling of a coating layer, resulting in separators with insufficient adhesion to electrodes and with insufficiently formed fine pores. It has also been found that because the poor solvent is not optimized in the above method (2), sufficient fine pores may not be formed in the coating layer. In the above method (3), because only a good solvent is used, the control of pore diameters of the porous layer is likely to be difficult, resulting in separators with insufficient permeability.
Microporous polyolefin membranes characterized in fine pore structures are also used as separating membranes for gas-gas separation, liquid-liquid separation, solid-liquid separation, etc. Separating membranes include uniform membranes entirely having fine pore structures, non-uniform membranes having fine pore structures on or inside the membranes and coarse pore structures supporting the fine pore structures, and composite membranes comprising a microporous membrane layer and a porous support layer, etc., and they are properly selected depending on substances to be separated. Recently, microporous separating polyolefin membranes are required to have improved separating performance and mechanical strength.
Thus, JP6-198146A proposes a microfiltration membrane comprising two microporous layers, one microporous layer being thinner and finer in pores than the other microporous layer. This microfiltration membrane is produced by (1) a method of coating a microporous membrane support with a polymer composition solution, and immersing the coated microporous membrane support in a liquid miscible with a solvent in the above solution but immiscible with the polymer composition, and then solidifying the polymer composition, or (2) a method of simultaneously extruding two polymer composition solutions with different polymer compositions or concentrations to form a laminate, and then solidifying it. In any of the above methods (1) and (2), however, two microporous layers are easily peeled as described above.